The present invention relates to rotary electric machines such as turbine generators and, more particularly to locking key grooves in a rotor retaining ring for reducing maximum stress acting on the retaining ring.
A retaining ring is one of the most highly stressed and thus critical components present in a rotary electric machine such as turbine generator, motors or condenser. The main function of the retaining ring is to retain the field winding extending beyond each end of the rotor body against centrifugal forces acting upon the winding. For a rotor body mounted retaining ring, which is the most prevalent retaining ring design, the retaining ring is shrunk on, and keyed in place to, a portion of the rotor body at the inboard end of the ring, and to a centering ring at the outboard end of the ring. These fitments at the inboard and outboard ends of the retaining ring must be designed with sufficient interference to keep them tight up to 120% of rated speed. This forces the retaining ring to remain cylindrical and prevents differential movement in the tangential direction of the ring with respect to the rotor body. However, the maximum permissible interference is limited by the stresses introduced in the components and the temperatures required assembling or disassembling the rings. The locking keys are used at opposite ends of the retaining ring to take thrust loads and prevent relative axial motion between the rotor and the retaining ring.
Retaining ring stresses arise from: (1) the centrifugal forces generated by the payload; (2) the mass of the retaining ring itself; (3) the shrink fit made during assembly; and (4) the thrust loads acting on the sides of the locking key grooves. During normal operation, the centrifugal load of the end winding contributes 5,000 to 8,000 pounds for each pound of copper under the ring. This produces a high hoop stress, which, for two-pole rotors, attempts to stretch the ring into a slightly elliptical shape. The stress due to the retaining ring mass is approximately proportional to the square of the ring diameter and the material density. The axial forces result from the radial expansions of the retaining ring and rotor at a high rotating speed, and differential thermal expansion between the retaining ring and winding. The axial load acting on a locking key could range from 200,000 to 1,000,000 pounds, depending on the machine rating and size. In order to sustain such high loads and to minimize energy loss in retaining rings, a nonmagnetic, high-strength stainless steel (e.g., 18Mn-18Cr) has been selected as the retaining ring material by the assignee since the mid-1980s.
Current locking key grooves are machined at the opposite ends of the retaining ring, facing radially inwardly. Conventionally, two half-circle slots, with equal radii, are machined at the corners at the base of the groove to prevent possible interference between the locking key and the retaining ring (see FIG. 2). A small platform formed in the base, between the two half-circles, is used to sustain the radial load from the locking key during normal operation. An analysis has revealed that maximum stress in a retaining ring is very sensitive to the radii of these half-circle slots. During normal operation, and as mentioned above, the locking key is used to sustain the axial thrust load from the retaining ring and to prevent relative axial motion to the rotor.
As a highly localized effect, stress concentration is usually introduced by an abrupt change in shape of a member, for example, notches and holes. For retaining rings, stress concentration regions usually occur at the bottom or base of the locking key groove, and more particularly, near the half-circle slots due to geometric discontinuity.
Redesign of the locking key grooves can alter the stress distribution in the retaining ring and thus improve stress concentration near the half-circle slots. Two alternative designs are proposed herein. In a first embodiment, the groove height (i.e., depth) and width remain the same as in the conventional design, but each corner slot is made by a set of curves, consisting of two quarter-circles and a short straight line between them. A flat but radially offset platform remains in the bottom or base of the groove, between the two slots.
In a second embodiment, the individual slots and the small platform are completely removed from the bottom or base of the groove. In this design, the reduction of maximum stress is achieved from both the improved structure continuity and increased effective retaining ring thickness. It is found that up to 18% of the effective retaining ring thickness can be increased without changing the groove height and width. To prevent the interference between the locking key and the groove, the radii at the corners of the key radius must be larger than the radii at the bottom or base corners of the groove.
Accordingly, in its broader aspects, the invention relates to a retaining ring for a rotor assembly, comprising a ring body having inboard and outboard ends, each of the inboard and outboard ends having a locking key groove open in a radially inward direction, each locking key groove having a base and a pair of side walls, the base having at least one flat portion therealong representing a maximum depth of the groove.